1. Technical Field
The present invention relates to an image stabilization circuit used in an image capturing device or the like.
2. Related Art
In recent years, image capturing devices such as a digital still camera and a digital video camera realize higher image quality by increasing the number of pixels of an image capturing element of the image capturing device. On the other hand, as another method for realizing higher image quality of the image capturing device, it is desired to equip the image capturing device with an image stabilization circuit having a shake correction function in order to prevent shaking of an imaging target caused by shaking of the hand holding the image capturing device.
More specifically, the image capturing device comprises a detecting element such as a gyro sensor, and an optical component such as a lens and an image capturing element according to an angular velocity component caused by vibration of the image capturing device, to prevent shaking of the imaging target. With this structure, even when the image capturing device is vibrated, the component of the vibration is not reflected in the acquired image signal, and a high quality image signal without image shake can be acquired.
FIG. 5 is a functional block diagram of an image stabilization circuit. An image stabilization circuit 100 comprises an analog-to-digital converter circuit (ADC) 10, an adder circuit 12, a servo circuit 14, a high-pass filter (HPF) 16, an integrator circuit 22, a centering processor circuit 24, and a digital-to-analog converter circuit (DAC) 26.
The image stabilization circuit 100 is connected to a position detecting element 102, a lens driving element 104, and a vibration detecting element 106. The position detecting element 102 is provided on at least two or more axes so that the position of a lens driven by the lens driving element 104 can be measured in such a manner as to at least allow orthogonal conversion. Similarly, the vibration detecting element 106 is provided on at least two or more axes so that the components of the vibration can be orthogonally converted along two axes including a yaw direction and a pitch direction. Output signals of the position detecting element 102 and the vibration detecting element 106 are subjected to an addition process or the like between X-axis components and between Y-axis components, and the lens position is controlled in the yaw direction (X-axis direction) and the pitch direction (Y-axis direction) based on the processed output signals.
The ADC 10 converts an analog voltage signal which is output from the position detecting element 102, for example, a Hall element, into a digital signal. The Hall element generates an induced current corresponding to a magnetic force from a magnet which is fixed on the lens, and outputs a voltage signal indicating the position of the lens according to the induced current. The ADC 10 converts the voltage signal into a digital signal, and outputs the converted signal as a position signal (Hall-X, Hall-Y). The ADC 10 is configured such that the ADC 10 outputs a signal which indicates a reference, for example, a digital value of “0”, when the optical axis of the lens and the center of the image capturing element provided in the image capturing device match each other. The ADC 10 also converts an analog angular velocity signal (Gyro-X, Gyro-Y) which is output from the vibration detecting element 106, for example, a gyro sensor, into a digital signal. More specifically, the ADC 10 digitizes the output signals from the position detecting element 102 and the vibration detecting element 106 in a time divisional manner and outputs the resulting signals. The ADC 10 outputs the signals (Gyro-X, Gyro-Y) to the HPF 16 and the signals (Hall-X, Hall-Y) to the adder circuit 12.
The HPF 16 removes a direct current component included in the angular velocity signal which is output from the vibration detecting element 106, and extracts a high-frequency component of the angular velocity signal reflecting the vibration of the image capturing device. The integrator circuit 22 integrates the angular velocity signal which is output by the HPF 16, and generates an angle signal which indicates an amount of movement of the image capturing device. The integrator circuit 22 preferably comprises a digital filter (not shown), and determines the angle signal, that is, the amount of movement of the image capturing device, by applying a filtering process according to a filter coefficient which is set in a register (not shown).
As shown in FIG. 6, the centering processor circuit 24 comprises a high-pass filter (HPF) comprising a low-pass filter (LPF) 24a which allows only a frequency band of the input signal which is less than or equal to a predetermined frequency to pass, a latch circuit 24b which latches an output value of the LPF 24a according to a latch control signal and outputs the latched value, and an adder 24c which outputs a difference between an input signal and the output value of the latch circuit 24b. When a shake correction process is executed in the image capturing device, there may be cases where, as the correction process continues to be executed, the position of the lens is gradually deviated from the reference position and reaches a point near a limit point of the movable range of the lens. If the shake correction process is continued, the lens can move in one direction, but cannot move in the other direction. The centering processor circuit 24 is provided in order to prevent this phenomenon, and controls the lens so that the position of the lens does not tend to reach the limit point of the movable range, by subtracting a predetermined value from the angle signal.
The adder circuit 12 adds the position signal (Hall-X) which is output from the ADC 10 and an X-axis component of a vibration component signal (SV-X) generated by the centering processor circuit 24, adds the position signal (Hall-Y) which is output from the ADC 10 and a Y-axis component of the vibration component signal (SV-Y) generated by the centering processor circuit 24, and outputs the results to the servo circuit 14. The servo circuit 14 generates a correction signal SR for controlling the driving of the lens driving element 104 according to the output signals from the adder circuit 12. The servo circuit 14 comprises a register and a digital filter circuit, and applies a filter process using a filter coefficient stored in the register. The DAC 26 converts a digital correction signal SR into an analog signal. The lens of the image capturing device is driven in each of the X-axis direction and the Y-axis direction by the lens driving element 104 based on the correction signal SR which is converted into an analog signal by the DAC 26.